It is known in the art to use synthetic, elastic, joint implants made of non-biodegradable plastics to replace damaged tissue joints, particularly joints connecting between small bones in the hands and feet. A typical biostable elastic joint prosthesis is composed of a spacer portion, which is positioned between the bones to be joined, and two elongated fixation portions, which are anchored in the bones to be joined. Such an artificial joint is available, for example, from Dow Corning, S.A., Valbourne Cedex, France, under the trade name Silastic.RTM.. The Silastic.RTM. implant is made of an elastomer material.
However, the use of joint prostheses manufactured of biostable polymers, polymer mixtures and elastomers can cause problems for the patient. One such problem with biostable joint prostheses is that the operated limb can only withstand a set amount of strain following the operation. For example, when a Silastic.RTM. joint prosthesis is used to replace a finger joint, the operated hand cannot bear a strain of more than 5 kg-force, and over-straining may lead to breaking or wearing of the implant forming the joint prosthesis. Another problem of such biostable joint prosthesis is that loose particles may be released from the joint prosthesis, due to wearing, fatigue and/or corrosion, which particles may cause a chronic inflammation reaction, e.g., a synovitis, and/or osteological changes in the bone. Further, the inflammation reaction may cause tumefaction and pain in the joint, often to a degree which requires removal of the joint prosthesis.
PCT publication WO 96/41596, which is assigned to the assignee of the present application, describes a biodegradable joint prosthesis that is comprised of a spacer part and proximal and distal fixation parts, which are fixed to the bones to be joined. A joint prosthesis in accordance with WO 96/41596 can be implanted in the hand, wrist or foot area to entirely or partially replace a damaged joint. The spacer part of the joint prosthesis of WO 96/41596 keeps the bones to be joined at a desired distance from each other. The joint prosthesis is fixed to the bones by anchoring the proximal and distal fixation parts, respectively, in the two bones to be joined. The strength and ductility values of the fixation parts of the joint prosthesis of WO 96/41596 are chosen to allow bending of the operated joint shortly after implantation.
The spacer part of the joint prosthesis of WO 96/41596 is important to the function of the prosthesis. In order to obtain a desired porosity (typically, a pore size of between 50 .mu.m and 1000 .mu.m), the spacer part of WO 96/41596 is preferably formed of a fibrous, three dimensional, partly porous structure of biodegradable fibers, which fibers have a typical thickness of between 1 and 300 .mu.m. To produce that spacer part, the biodegradable fibers can be woven or knit or formed into a non-woven fabric and, then, folded or wrapped into a three-dimensional, pillow-like, porous structure. The spacer part of WO 96/41596 can also be produced from: (I) a continuous fiber blank using three dimensional weaving, knitting or twisting techniques, wherein a desired length of the continuous fiber blank is cut for each spacer part, (ii) biodegradable fibers which are cut into desired lengths and are bound together using a biodegradable binding blank; or (iii) a biodegradable, continuous, polymer blank which is treated to produce open porosity therein.
The shape of the spacer part of WO 96/41596 preferably corresponds to the space between the bones to be joined by the joint prosthesis. Accordingly, that spacer part may have the shape of a pillow, a cylinder, an ellipsoid, (flattened) ball, cubic or rectangular prism, or other three dimensional structure which, as well as possible, fills the space constructed for the spacer part between the bones to be joined. The spacer part of WO 96/41596 is advantageously elastic, and it may not contain any sharp and/or hard edges or angles which could cause mechanical irritation of the surrounding tissue.